Constructing stations for Copenhagen’s new metro line is leading to some innovative piling solutions, reports Claire Symes
Denmark’s capital city is currently home to a large fleet of piling rigs and hydromills which are playing an essential part in delivering a new metro line for Copenhagen - Cityringen - by 2018. But due to noise, vibration and working time restrictions placed on ground engineering contractor Trevi, it would be easy for the work these rigs are doing to pass unnoticed by the majority of the population.
Cityringen involves construction of a twin tunnelled 15.5km circular metro line around the centre of Copenhagen with 17 stations, including interchanges with the existing underground network, which will link areas not already served by the city’s S-trains or metro system.
Trevi is the main subcontractor working on the DKK21.3bn (£2.3bn) scheme for main contracting joint venture CMT (Cityringen Metro Team), formed from Italy’s Salini, Seli and Tecnimont.
Trevi is undertaking all the main foundations work, which mainly involves a mix of diaphragm walling and Trevi’s cased secant piling (CSP). But the £81M contract may be extended to include compensation grouting when the tunnel boring machines (TBMs) are launched.
“Eventually we will be working on 21 construction sites - it is a big challenge,” says Trevi project manager Giancarlo Zannoni.
The geology is an issue with glacial till formed from sand and boulders to 10 to 12m depth. Below that is limestone - the upper layers are weak but it becomes harder with depth and the flints are very hard, with strengths of up to 600MPA.
“We have had to replace 500 teeth on the hydromills so far,” says Zannoni.
Trevi geotechnical engineer Paulo Cavalcoli adds: “The previous metro lines in Copenhagen were constructed through use of diaphragm walls built using grabs and chiselling through the flint layers. The noise from this caused some problems, which is why air hammering techniques are not allowed on this scheme. “The other two lines are not as deep as the new system that we are building, which adds to the challenge on this project.”
CSP is a new technique for Denmark - it is basically a fully cased continuous flight auger pile, but Cavalcoli says it uses no polymer or bentonite so is less messy. “Here all the piles extend to 28m and we are using tools that were specially developed for the scheme and it is a design that we are still tweaking,” he says. “The flint is highly abrasive so we are using a high quality steel to be more productive and adjusting the tooling position to create a more aggressive auger.”
Trevi is working on 12 sites at the moment. Two - Nørrebroparken and Nørrebros Runddell - are already finished.
Trevi finished at Nørrebroparken in June and work on casting the top slab and excavation of the box has been completed. The first of the project’s earth pressure balance TBMs was launched in September from Nørrebroparken and will travel anticlockwise towards Nørrebros Runddell.
Main contractor CMT will eventually have four 6m diameter Kawasaki earth pressure balance TBMs working on the scheme and the second is due to be launched soon from Otto.
Zannoni says that CMT hopes to achieve 13 to 14m of tunnel construction per day.
Trevi has 200 people working on the scheme and this has gradually ramped up since the company started in August 2011.
One of Trevi’s most challenging sites is Sønder Boulevard. The actual foundation design is fairly straightforward, but the work is complicated by contaminated soiland restricted working hours.
The new station is being constructed using diaphragm walling, and Trevi is using two hydromills and two cranes to construct the station box. But instead of just one desanding plant, Trevi has had to squeeze two onto the constricted site - one to deal with the natural ground and the other to deal with hydrocarboncontaminated ground.
The station calls for 68 diaphragm wall panels to be installed to 28m to form a 15m wide, 90m long box. “The panels are 2.8m wide and 1.2m thick,” says Sønder Boulevard site manager Francesco Ricci. The verticality of the panels is checked using on-board systems and selectedpanels are checked using an ultrasonic system.
Primary and secondary panels are being installed in a careful sequence with at least five days between adjacent panels. “It is taking three to four days per panel - work is slower due to the contamination which lies at between 11 and 20m below ground level,” explains Ricci.
Margozzi adds: “The benzene pollution is believed to have been caused by use of this area by an oil refinery in the 1920s. Because we are working close to residential properties, we have had to take special precautions. The desanding plant has been covered to protect the area from pollution rather than noise and dust and a monitoring system has been installed in the covered plant to warn if pollution levels reach a critical level.”
“We have 10-minute toolbox talks every day to remind staff about the risks on this site,” says Margozzi.
Noise is also a concern. “The site is in a very quiet area of Copenhagen and there are strict noise limits placed on our work,” says Margozzi. “Between 7am and 6pm we are limited to 70dB(A) but between 6pm and 7am and all day on weekends, we are limited to just 40dB(A). Normally on this kind of scheme we would work 24 hours a day but these limits make this impossible - most sites are operating 10-hour shifts - and we have had to look at our equipment to make sure we meet demands during the daytime too.”
Part of the solution has been to use sound-reducing fencing around the sites, efforts have been made to insulate the Soilmec machines being used on the project, and mobile sound barriers are also being used to further reduce noise levels. “We are monitored 24 hours a day by the client for noise and vibration levels,” says Margozzi.
As Zannoni has already mentioned, the ground conditions across Copenhagen also present some challenges - the geology is mainly limestone with a flint layer that can be up to 2m thick. Trevi is overcoming the problem by predrilling using a sonic drill to “pre-fracture” the flints to try to reduce wear on the piling auger and damage to the hydromill.
Ricci says: “The sonic drill is being used to predrill at all locations at Sønder Boulevard with five, 120mm diameter bores being drilled to 14m at every panel location.”
In a city where the canals are a major tourist attraction, it is hardly a surprise that water levels are high - generally 1 to 2m below ground. Protection of water quality is therefore a challenge too.
“Any water extracted to dewater the work has to be treated before it can be discharged,” says Cavalcoli. “A lot of the buildings in Copenhagen have wooden piled foundations, so we are only allowed to drop water levels outside the sites by 50mm to prevent damage to the foundations.”
Dewatering is being managed by specialist contractor Hoelscher from Germany.
But there are also environmental concerns associated with the work and Trevi has had to specially prepare machines for this project. “We are using vegetable oil in our machines and all of them are equipped with spill kits should there be a hydraulic oil leak,” says Margozzi.
All the machines are new and fitted with Soilmec’s drill monitoring system (DMS), which is being monitored from the main office Trevi shares with CMT. The rigs have also been fitted with particulate filers to reduce engine emissions. “The rigs all meet EU requirements but the client has decided to set a higher standard,” says Margozzi.
If all this wasn’t enough to deal with, winters in Copenhagen are also cold, which has an impact on the bentonite, concrete and PPE needed. “The days are also very short,” adds Margozzi.
While the piling and diaphragm walling work is getting into a good rhythm, there are still some further challenges for Trevi - CMT is still considering the use of grouting in some areas to manage settlement from the TBM, and other techniques are also being called for on some stations.
“At Gamestrande we are going to use micropiling techniques for the temporary works to construct a slab over the canal from which we can install the diaphragm wall,” explains Zannoni. “The station calls for us to work in the middle of the canal channel but we have to ensure boats can continue to use the canal throughout the work.”
Trevi’s work is scheduled to finish in mid-2014. “We will complete the piling work by the end of 2013 though,” says Zannoni.
One of the most high profile sites is the station at Marmorkirken where Trevi is working just 100mm from a historic church, which has just undergone restoration.
The station takes its name from the church, which was built between 1749 and 1900, and at 31m span, is the largest domed church in Scandinavia.
The work involves construction of a T-shaped diaphragm wall for a new station and what was previously the church gardens, which will be restored at the end of the project.
In total, 116 diaphragm wall panels will be installed at the site, to 28m below ground level.